python - i have '__contains__' ,why error -

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class a(object):     def a(self):         return true     __contains__=a  b=a() print 2 in b#why error

__contains__ meant take argument. a doesn't accept argument.

the following example working __contains__:

>>> class a(object): ...     def a(self, item): ...         return true ...     __contains__=a ... >>> b=a() >>> print 2 in b true

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